1. Technical Field
The present invention generally relates to electrical driving systems and driving current switching in such driving systems.
2. Description of Related Art
Referring to FIG. 10, this shows a conventional fan driving system 10 adapted to drive a fan 101. The driving system 10 and fan 101 are used in a computer. The fan driving system 10 includes a driving device 11 and a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) group 12. The MOSFET group 12 includes a first MOSFET (i.e., MOSFET1), a second MOSFET (i.e., MOSFET2), a third MOSFET (i.e., MOSFET3), and a fourth MOSFET (i.e., MOSFET4). The driving device 11 is configured for receiving a set of Hall sensing signals N+ and N− from a Hall sensor (not shown) of the fan 101, and outputting a first driving signal VG1, a second driving signal VG2, a third driving signal VG3, and a fourth driving signal VG4 respectively to a gate G1 of the first MOSFET, a gate G2 of the second MOSFET, a gate G3 of the third MOSFET, and a gate G4 of the fourth MOSFET. Thereby, the MOSFETs in each pair of coupled MOSFETs of the MOSFET group 12 are selectably switched on simultaneously. That is, the first MOSFET and the fourth MOSFET are selectably switched on simultaneously, or the second MOSFET and the third MOSFET are selectably switched on simultaneously. Referring also to FIG. 11, after the first MOSFET with the fourth MOSFET are switched on and thereby enabled, a driving current If flows through the fan 101 via an electrical loop constituted by the driving device 11, the first MOSFET and the fourth MOSFET. Referring also to FIG. 12, the second MOSFET and the third MOSFET are then switch on and thereby enabled, and another driving current Ib flows through the fan 101 via an electrical loop constituted by the driving device 11, the second MOSFET and the third MOSFET. The driving current Ib flows in a direction opposite to the direction in which the driving current If flows. The driving currents If and Ib alternately flow through the fan 101, thereby driving the fan 101 to rotate.
A rotational speed of the fan 101 is determined by the levels of the driving currents If and Ib. In particular, the higher the levels of the driving currents If and Ib, the faster the rotational speed of the fan 101. However, the relationship between the levels of the driving currents If and Ib and the rotational speed is non-linear. Therefore, in order to control the rotational speed of the fan 101, it is necessary to appropriately regulate the levels of the driving currents If and Ib. A conventional mode for regulating the driving currents If and Ib is a PWM (Pulse Width Modulation) driving mode, wherein the rotational speed of the fan 101 can be regulated by simply controlling a pulse width of the PWM waves corresponding to the driving currents If and Ib.
A noise usually is generated while the fan 101 rotates, due to the low switching frequency between the driving currents If and Ib. Accordingly, in order to suppress the generation of noise, one approach is to set the switching frequency between the driving currents If and Ib to be higher than 25 kilohertz (KHz).
Typically, during the process of switching the driving currents If and Ib, a residual current stored in a coil of the fan 101 cannot be fully discharged in time. This is due to all of the first MOSFET, the second MOSFET, the third MOSFET and the fourth MOSFET being switched off. Therefore, at the moment of switching, a peak voltage in the coil of the fan 101 is excessively high. This is liable to result in the fan driving system 10 being damaged and disabled.
What is needed, therefore, is a fan driving system for a fan which can overcome the above-described shortcomings.